Batteries are electrochemical cells designed to store and release energy. For simplicity, the term “battery” is used herein to refer to energy storage devices comprising one or more electrochemical cells configured to provide an output voltage and/or electric current. Primary batteries convert chemical energy to electric work in a single discharge, while secondary or rechargeable batteries may be discharged and charged multiple times. Due to the ubiquitous need for energy storage and the limitations arising therefrom, improved batteries enable advancements in numerous fields of technology and human interest.
Exemplary state-of-the-art rechargeable battery technologies include lead-acid, Ni-MH and Li-ion batteries. The theoretical potential for performance improvements in these mature technologies is limited. In particular, practically achieved capacities are nearing theoretical limitations defined by the positive and negative electrode active materials. Consequently, novel electrode materials and cell chemistries are sought that provide significantly higher theoretical specific energy and energy density.
The aforementioned rechargeable battery technologies utilize either aqueous or organic electrolytes and generally operate at or near ambient temperature. For certain applications, active thermal management is required for these systems to maintain battery temperature near 25° C., since deviations from this temperature regime lead to performance losses, cell failure and/or safety hazards. In contrast, molten salt batteries operate at an elevated temperature above the melting point of an inorganic molten salt electrolyte. Primary molten salt batteries, often referred to as “thermal batteries”, are stored in the solid state and are heated to activate for a single discharge. Storage in an inactive solid state prevents self-discharge, thus allowing very long shelf life. An exemplary commercial primary thermal battery utilizes a Li alloy anode, a molten alkali metal halide electrolyte and a cathode comprising FeS2 active material. While commercially successful as a primary thermal battery, rechargeable Li alloy-FeS2 cells exhibit inferior performance attributes compared to, for example, Li-ion batteries, due largely to issues related to the high (>300° C.) operating temperature imposed by the high liquidus temperatures of alkali metal halide eutectics
The “ZEBRA” battery, which comprises a molten Na metal negative electrode and a transition metal halide (e.g. NiCl) positive electrode, provides an example of a commercially successful rechargeable molten salt battery. In this system, negative and positive electrode compartments are separated by a solid Na+ conducting beta alumina solid electrolyte (“BASE”) membrane. The positive electrode compartment further comprises a molten salt electrolyte composed of NaAlCl4, which has a melting point of 167° C. ZEBRA batteries typically operate at >245° C., a temperature which is imposed by transport and interfacial properties of the BASE membrane. State-of-the-art commercial ZEBRA-type batteries achieve >3000 cycles and practical specific energy and energy density of ˜115 Wh/kg and ˜160 Wh/l, respectively. However, ZEBRA batteries are still encumbered by issues related to the high operating temperature, including long startup times, poor stability to thermal cycling and safety concerns, which limit their practicality for certain applications, particularly electric vehicles.
Molten salt batteries that are capable of lower temperature operation (e.g. <200° C.) are an attractive target of research and development, since it is contemplated that this lower temperature may still confer improved electrochemical properties, such as fast electrode kinetics and charge transport, while lessening problems and materials constraints associated with the higher temperature regime (e.g. polymeric seals may be used instead of ceramic to metal seals). Alkali metal nitrates and nitrites are a class of inorganic molten salt that have been investigated for use as “intermediate temperature” molten salt electrolytes, since they can be formulated in eutectic mixtures with melting points below 100° C. Alkali metal nitrates and nitrites provide additional beneficial characteristics motivating their consideration for both primary and rechargeable molten salt batteries. Some of these properties include high thermal stability (>500° C.), stability in contact with air, low viscosity, low cost, low corrosiveness to typical container materials, little to no volatility and high heat capacity.
Moreover, alkali metal nitrates exhibit unique electrochemical properties which previously have been proposed in various connections related to energy storage applications. For example, the NO3− anion is reduced by Li metal according to the following reaction:2Li+LiNO3→LiNO2+Li2O  1)This reaction generates a passivating film, or solid-electrolyte interphase (SEI), on the surface of Li metal composed of Li2O which is permeable to Li+ cations but inhibits continuous reaction with the electrolyte. This SEI phenomenon has enabled research and development of batteries employing molten nitrate electrolytes in direct contact with Li metal or Li alloys. A variety of positive electrode materials have been disclosed for use in such batteries including, for example, soluble transition metal cations, transition metal oxides and, more recently, gaseous O2 (see U.S. Pat. Nos. 4,416,958, H1,544, and 6,544,691, each of which are incorporated by reference herein in their entireties).
The oxidizing NO3− anion has also been disclosed for use as the positive electrode active material in molten salt batteries (see U.S. Pat. Nos. 4,260,667 and 4,535,037, each of which is incorporated by reference herein in their entireties). According to this concept, the battery discharge process involves oxidation of a metal negative electrode and reduction of the NO3− anion component of the molten salt electrolyte on the positive electrode surface. In the case of Li metal batteries, the positive electrode reaction is:2Li++LiNO3+2e−→LiNO2+Li2O  2)At 150° C., the thermodynamic potential of this reaction is ˜2.5 V vs Li+/Li and the theoretical specific capacity is 646 mA/cm2, equating to a high specific energy of 1615 Wh/kg.
Due to substantial irreversibility, this positive electrode has heretofore been considered for use in primary thermal batteries and has not been demonstrated in a rechargeable battery. The object of the present invention is to provide catalytic materials that enable highly efficient cycling and long cycle life of cells employing NO3− as a positive electrode active material by catalyze reversible formation of NO3− from NO2− and O2− during battery charging. This objective is motivated by the need for rechargeable batteries with improved performance (e.g. specific energy and energy density) compared to state-of-the-art rechargeable batteries (e.g. Li-ion).